Plasma torch electronic pulsing circuit

ABSTRACT

A plasma arc torch having new electronic circuit concepts wherein main current regulated power means regulates the pilot current prior to main arc transfer. Further, the circuit may contain two inductors to which DC current initially flows but is interrupted upon main arc transfer such that one inductor maintains the pilot arc while the current in the second inductor forces the establishment of the transferred arc. Also, advantages are presented in pulsing the cutting arc as well as pulsing the pilot arc.

CROSS-REFERENCE TO RELATED APPLICATION

The subject matter of this application is related to and comprises acontinuation-in-part of the patent application having Ser. No.07/682,727, filed on Apr. 8, 1991 pending now U.S. Pat. No. 5,189,277,which application is owned by a common assignee.

BACKGROUND OF THE INVENTION

The present invention is in the field of plasma torches and inparticular is directed to a plasma torch having an improved pilot andmain arc generating circuit.

Plasma torches, otherwise known as electric arc torches, are known inthe art for performing operations, such as cutting, welding, etc , onworkpieces, and operate by directing a plasma consisting of ionized gasparticles towards a workpiece. An example of the conventional single gasplasma torch is illustrated in Hatch, U.S. Pat. No. 3,813,510, assignedto the assignee herein Other patents disclosing such torches are U.S.Pat. Nos. 4,225,769; 4,663,512; and 4,663,515. The disclosures of all ofthe above-mentioned patents are incorporated herein by referencethereto. As these patents illustrate, a gas to be ionized, such asnitrogen, is fed through channels in the torch mechanism in such amanner as to swirl in front of the end of a negatively chargedelectrode. The welding tip which is adjacent the end of the electrodehas a sufficiently high voltage applied thereto to cause a spark gap tojump between the electrode and the welding tip, thereby heating the gasand causing it to ionize. A pilot DC voltage between the electrode andthe welding tip maintains the pilot arc. The iononized gas in the gapappears as a flame and extends externally of the tip where it can beseen by the operator. The extension of the pilot arc and the flame,which for practical purposes, may be considered as being co-extensivedepends upon the power in the gap—i.e., the arc current—as well as thepressure of the gas forced into the gap and out of the torch. The pilotarc provides a source of light which enables the operator to see theproper position for the torch before starting the welding or cuttingoperation. In actual practice, when the pilot arc is on, a loop-shapedarc extending out of the torch can be seen. As the torch head is broughtdown towards the workpiece, the pilot arc jumps from the electrode tothe workpiece due to the fact that the impedance of the workpiececurrent path is lower than the impedance of the welding tip currentpath.

Conventional single gas plasma torches include pilot arc circuits whichprovide a 20-40 amp. pilot arc current at 100-200 volts across theelectrode-tip gap, resulting in an extension of the arc about ¼-½ inchpast the welding tip. As a consequence, the torch must be brought towithin about ¼-½ inch of the workpiece before the transfer arc jumps tothe workpiece. This creates difficulties in the starting of cutting orwelding operations.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a plasma arc torch circuitwhich is more efficient than prior art circuits and which regulate thepower source in response to sensing of the torch arch arc transferringfrom between the pilot electrode and the torch electrode, to torchelectrode and the work member.

It is another object of the invention to optimize the power regulatordesign with respect to semiconductor switch and diode stresses,transformer and inductor design and overall component count.

It is still a further object to provide a plasma arc torch with smootheron-plate transfer dynamics

Briefly stated, the present invention comprises new electronic circuitconcepts for a plasma arc torch wherein main current regulated powermeans regulates the pilot current prior to main arch transfer Further,the circuit may contain two inductors to which DC current initiallyflows but is interrupted upon main arc transfer such that one inductormaintains the pilot arc while the current in the second inductor forcesthe establishment of the transferred arc. Also, advantages are presentedin pulsing the cutting arc as well as pulsing the pilot arc.

These, as well as other objects and advantages will become more apparentupon a reading of a detailed description of the preferred embodiment inconjunction with the drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic wiring diagram of one prior art plasma arc torchoperating circuit;

FIG. 2 is schematic wiring diagram of another prior art plasma arc torchoperating circuit;

FIG. 2A is a schematic wiring diagram of a prior art pilot arcregulating circuit as used within FIG. 2;

FIG. 2B is a schematic wiring diagram of another prior art pilot arcregulating circuit as used within FIG. 2;

FIG. 3 is a schematic wiring diagram depicting a plasma arc torchoperating circuit according to the principles of the present invention;

FIG. 4 is a schematic wiring diagram of a modified embodiment of thepresent invention as it would be configured within the circuit of FIG.3; and

FIGS. 5A and B, in combination, are a more detailed schematic wiringdiagram of the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawings, and FIG. 1 in particular, there is showna schematic wiring diagram for one prior art plasma arc torch, generallyreferred to by the reference numeral 10. Torch 10 includes torch tipelectrode 12or torch electrode as is known in the art and annular torchpilot electrode 14or tip as is known in the art spaced from tiptorchelectrode 12. An electronic pilot circuit P connected between tip torchelectrode 12 and pilot electrode 14 provides an electric potentialbetween electrodes 12 and 14 to create a pilot arc which heats asupplied gas such as nitrogen causing it to ionize as is well known inthe art. FIG. 1 shows prior art circuit C which uses a resistivelyregulated pilot arc having a current regulated power means 16 and apilot regulator means 18 including a disconnect means 20 in series witha resistor 22. A high frequenty pilot initiation means 24 is positionedin series with pilot regulator means 18 and may be inserted in thecircuit adjacent either electrode 12 or electrode 14 as shown in FIG. 1to initiate investigation ionization of plasma gas to commence pilotoperations.

A current sensing means 26 is connected in parallel with pilot regulatormeans 18 and connects with the metal to be worked at series with work 28in main circuit M. When tip torch electrode 12 is placed sufficientlyclose to the metal work 28 the arc will transfer to the work 28 causingcurrent to flow through main circuit M and current sensing means 26 willsense the current differential and act to disconnect pilot regulatormeans 18 by opening pilot regulator means 18 by opening its disconnectmeans 20.

One problem associate with the prior art circuit 10 of FIG. 1 is thatthe circuit voltage of current regulated power means 16 must be largecompared with the torch piloting voltage between tip torch electrode 12and pilot electrode 14 to allow the pilot regulator means 18 to performthe function of a current source during pilot opration. This causescircuit 10 to be inefficient, power being dissipated as heat in pilotregulator means 18.

FIG. 2 shows another prior art circuit 10′ similar to that of FIG. 1 inthat it also contains electronically controlled pilot regulator means18′ in pilot circuit P′ and a parallel main circuit M′. The circuit 10′of FIG. 2 also includes similar current operated power means 16′, tiptorch electrode 12′, pilot electrode 14′, current sensing means 26′,pilot initiation means 24′ (alternatively positioned as shown) and workpiece or member 28′. The difference between circuit 10′ of FIG. 2 andcircuit 10 of FIG. 1 is the provision of circuit connection on theopposite side of power means 16 from current means 26′ connected topilot regulator means 18′ in order to provide a second current regulatedcontrol loop, one for pilot arc operation and one for transferred mainarc cutting that FIG. 2 uses an active loop with feedback, either linear(FIG. 2a) or switching (FIG. 2b) regulator, while FIG. 1 uses a passivecurrent limiting means (resistor).

FIG. 2A shows one prior art pilot regulating circuit 18′a wherein thepilot is linearly regulated; that is, the pilot current is regulatedagainst a set demand means 3d by varying the conductance of a linearelement 3a.

FIG. 2B shows another prior art pilot regulating 18′b wherein the pilotis switch regulated. That is, the pilot is regulated against a setdemand 3d′ to vary the duty cycle of a switching element 3g within afeedback loop. Either scheme can tightly regulate the pilot arc againstAC line variations and against plasma gas in use, however, both addparts count and cost to the torch and are relatively inefficient.

FIG. 3 shows the preferred plasma torch circuit 100 in accordance withthe principles of the present invention. Circuit 100 includes torch tipelectrode 112, pilot electrode 114, current regulated power means 116,pilot regulator means 118, alternatively positionable pilot initiationmeans 124, current sensing means 126, and metal work 128. Pilotregulator means 118 comprises an electronic disconnect 120 in serieswith a current smoothing and energy storage inductor 130, and afree-wheeling diode 132 connected in parallel with disconnect 120 and inseries with inductor 130.

According to the invention, current sensing means 126 not only controlsdisconnect 120 through line 134, but also sends a current signal tocomparator 136 through line 138 which controls the output of power means116.

During torch piloting, disconnect means 120 is “on” and is in itssaturated state. The voltage seen between the metal work piece 128 andtorch tip electrode 112 is essentially the voltage at which the torchmaintains the pilot arc determined by torch geometry and the plasma gasused. This voltage is considerably lower than the open circuit voltageused in prior art torch circuits. When the torch is brought sufficientlyclose to metal work piece 128, ionization current is detected by currentsensing means 126. In response to sensing the working current, sensingmeans 126 acts through line 134 to force disconnect means 120 to its“off” or high impedance state. At the moment of arc trnasfer to workpiece 128, the pilot arc is maintained by current flowing through energystorage inductor means 130 and the free-wheeling diode means 132. At thesame instant, the current flowing in through the smoothing inductormeans 140 of power regulator 116 is forced to flow between the workpiece128 and torch tip electrode 112,. At the moment of disconnect,disconnect 120 open circuits the inductor means 140 to generate atransient voltage between the torch electrode and the workpiece usingthe stored energy in the inductor means 140, greatly in excess of thepilot voltage, that initiates and initially supports the transfer to themain arc, thereby maintaining the transferred plasma arc. When theenergy is dissipated in the storage inductor means 130, the pilot arcbetween torch tip electrode 112 and pilot electrode 114 selfextinguishes. When transfer is detected in current sensing means 126 thepilot demand means 1e is changed and the power means 116 changes thepower to that demanded for the torch operation on work piece 128.

A further embodiment of the present invention resides in additionallypulsing the pilot current.

Instead of maintaining a constant pilot demand means (1e), the demandmay be pulsed between two (or more levels) at various frequencies andduty cycles. During this pulsing the pilot arc is maintained throughoutand no high frequency arc initiation means 124 is required, as would bethe case for a ‘blown-out’ pilot.

This pulsing feature offers several advantages. First, higher standoffinstances between the work metal 128 and torch tip electrode 112 at themoment of transfer. Second, a tip cleaning action is observed i.e.during plasma cutting molten metal is blown onto the tip face where itadheres in particulate form. At the same time, electrode material isremoved from the torch electrode and adheres to the inside tip. Bothforms of contamination can cause the tip orifice to become distorted.When the pilot arc is pulsed following each cut significantly more poweris dissipated in the tip torch electrode 112 for the pulse duration.This thermal modulation is believed to be responsible for dislodgingmetal particles from the inner and outer tip surfaces.

With reference to FIG. 4, there is shown an alternative circuit whereina small resistor 142 is added in series with the pilot means 118. Thismodification can further improve the obtainable standoff on some plasmatorch designs. The pilot current (Ip) flows through resistor 142 togenerate a voltage drop (Ip×R) which is in series with the pilot voltagemeasured between torch tip electrode 112 and pilot electrode 114. Thusthe open circuit voltage between the metal work piece means 128 and thepilot electrode means 114 is increased, assisting the standoff attransfer. The power dissipated in this resistor is then a function ofthe pilot demand and pulse duration.

A further alternative circuit provides a pulsing cutting or main arc.From the invention pulsing the pilot arc before transfer it is clearthat it is possible to pulse the means 1e, after the arc has transferredand while the transferred plasma arc is cutting the work metal means128. This provision of pulsing the main arc offers several potentialadvantages. First, by selecting the appropriate pulse rate and dutycycle in relation to the cutting variables, it will offer aproportionally greater arc cutting capacity/penetration for a smallincrease in power consumption. Second, it allows the tip orifice size tobe reduced in comparison to a conventional plasma cutting systemoperating in response to a DC demand level. This will, allow, a smallerfocussed plasma column and result in smaller kerf widths. Plasma arcstability may also improve as a result of pulsing.

FIGS. 5A and B are a more specific electronic circuit schematic diagramembodying some of the concepts of the invention as enumerated above.Like reference numerals appearing in FIG. 5 refer to like circuitcomponents or group of components as appear in FIGS. 3 and 4. Referencenumberal 120c depicts the control circuity for disconnecting means 120.The power supply means is not shown in FIG. 5.

It can therefore be seen that the novel circuity shown in FIGS. 3through 5 fulfills the objects and provides the advantages set forthabove. Inasmuch as numerous changes could be made to the circitrywithout departying from the spirit and scope of this invention, thescope of the invention is to be determined solely by the language of thefollowing claims as interpreted by the patent laws and in particular thedoctrine of equivalents.

Having thus described the invention what is claimed and desired to besecured by Letters Patent is:
 1. A plasma torch unit of the type havinga torch assembly adapted to operate on a work member and having a torchelectrode, a pilot electrode, and means for providing an ionized gastherebetween, a current regulated power means electrically connectedbetween said electrodes and work member to supply regulated chargecurrent to the electrode electrodes and work member during torch usage,pilot arc control means for controlling an electric arc between saidelectrodes and pulsing the same during torch usage, said pilot arccontrol means electrically connected between said current regulatedpower means, the electrodes, and the work member, first circuitry of theelectrical connection provided between the power means and the workmember, second circuitry of the electrical connection provided betweenthe power means and the torch electrode, and third circuitry providedbetween the power means, pilot arc control means, and the pilotelectrode, disconnect means in said second third circuitry between thepower means and said pilot electrode, current sensing means for sensingcurrent in said first circuitry and generating a signal in response totorch usage, and an inductor means in series with said disconnect meansand the said pilot electrode, said current sensing means operating saiddisconnect means with a feedback signal to pulse the generated electricarc between the electrodes, and diode means connecting between saidsecond and third circuitry to regulate provide a path for the conduct ofcurrent to the pilot electrode.
 2. The torch unit of claim 1 and whereinsaid diode means providing for conduct of current from the currentregulated power means only towards the first inductor means totemporarily sustain the pilot current when said disconnect means isopen.
 3. The torch unit of claim 2 and further including resistor meansconnected between said first and second circuitry and generating avoltage drop related to the pilot demand and generated pulse durationthen increasing the electrode to workpiece voltage to increase thetransfer height.
 4. The torch unit of claim 1 and wherein a secondinductor means provided within the power means and responsive to thecurrent detected by the current sensing means to provide a pulsing ofthe generated main arc between the torch electrode and the metal workduring torch usage high frequency filtering of the power means.
 5. Thetorch unit of claim 4 and wherein said current sensing means generatinga signal to the second inductor comparator to regulate the currentgenerated by said power means and sustaining of the pulsing of the arcbetween the torch electrode and the work member.
 6. The torch unit ofclaim 5 and wherein the current generated of the current regulated powermeans also being pulsed by the second inductor .
 7. The torch unit ofclaim 6 and wherein said current sensing means operating to provide apulsed pilot arc.
 8. The torch unit of claim 6 and wherein said currentsensing means operating to provide a pulsed main arc.
 9. A plasma torchunit of the type having a torch assembly adapted to operate on aworkpiece, and having a torch electrode, a pilot electrode, and meansfor providing an ionized gas therebetween, the torch unit characterizedby: a current regulated power supply electrically connected between theelectrodes and the workpiece to supply regulated current to the torchelectrode and the pilot electrode for the pilot arc, and to the torchelectrode and the workpiece for the main arc; a pilot arc controlcircuit connected between said power supply and the pilot electrode andincluding a first pilot inductor and a diode that temporarily maintainspilot current when said pilot arc control circuit is open circuited; asecond inductor connected between said power supply and said torchelectrode; and a disconnect circuit selectively connected between saidpower supply and said pilot inductor and said diode for pulsing thegenerated electric arc between the electrodes such that when saiddisconnect circuit is open circuited said pilot inductor and said dioderemain connected between the torch electrode and the pilot electrodetemporarily supplying pilot current, said disconnect circuit connectedto said pilot arc control circuit and operable to open circuit saidpilot arc control circuit to generate an induced voltage in the secondinductor which is greater than the open circuit voltage thereby forcingcurrent to flow between the workpiece and the torch electrode to helpinitiate the transfer of the pilot arc into the main arc; wherein saidregulated current for the pilot arc is pulsed.
 10. A plasma torch unitof the type having a torch assembly adapted to operate on a work memberand having a torch electrode a pilot electrode, and means for providingan ionized gas therebetween; a current regulated power means forsupplying regulated current to the work member and torch electrodeduring torch usage, said current regulated power means electricallyconnected to the work member by first circuitry provided between thepower means and the work member and electrically connected to the torchelectrode by second circuitry provided between the power means and thetorch electrode; pilot arc control means for controlling an electric arcbetween the torch electrode and the pilot electrode, said pilot arccontrol means electrically connected between the power means and thepilot electrode and electrically connected to the pilot electrode bythird circuitry provided between the control means and the pilotelectrode, said pilot arc control means including disconnect meansbetween the power means and the pilot electrode; current sensing meansfor sensing current in said first circuitry and generating a feedbacksignal in response to torch usage; and diode means connecting betweensaid second and third circuitry to provide a path for the conduct ofcurrent to the pilot electrode; the unit characterized by: firstinductor means coupled to said pilot arc control means for temporarilysustaining the pilot current when said disconnect means is open; secondinductor means provided within the power means for forcing theestablishment of the transferred arc by way of an induced voltagegreater than the open circuit voltage when the disconnect means isopened thereby forcing current to flow between the workpiece and thetorch electrode; and an interconnect means between the current sensingmeans and the disconnect means so that said current sensing meansoperates said disconnect means with the feedback signal.
 11. The torchunit of claim 10 wherein said diode means provides conduct of currentfrom the current regulated power means only towards the inductor meansto temporarily sustain the pilot current when said disconnect means isopen.
 12. The torch unit of claim 11 further including resistor meanselectrically connected between said first circuitry and the disconnectmeans generating a voltage drop related to the pilot demand thenincreasing the electrode to work member voltage to increase the transferheight.
 13. The torch unit of claim 10 wherein the second inductor meansprovided within the power means is responsive to the current detected bythe current sensing means to provide high frequency filtering of thepower means.
 14. The torch unit of claim 10 wherein said current sensingmeans generates a signal to a comparator to regulate the currentgenerated by said power means.
 15. The torch unit of claim 10 whereinthe current generated by the current regulated power means is beingpulsed.
 16. The torch unit of claim 10 wherein said current sensingmeans operates to provide a pulsed pilot arc.
 17. The torch unit ofclaim 10 wherein said current sensing means operates to provide a pulsedmain arc.
 18. The torch unit of claim 10 wherein: the current regulatedpower means is electrically coupled between the electrodes and the workmember to supply regulated current to the torch electrode and the pilotelectrode for the pilot arc, and to the torch electrode and the workmember for the main arc; and the disconnect means comprises a disconnectcircuit coupled to said pilot arc control circuit and operable to opencircuit said pilot arc control circuit to generate an induced voltagewhich is greater than the open circuit voltage in a second inductormeans coupled between said power means and said torch electrode to helpinitiate the transfer of the pilot arc into the main arc.
 19. The plasmatorch unit of claim 18, wherein said pilot arc control circuit includesthe inductor means for temporarily maintaining pilot current when saiddisconnect circuit is open.
 20. The plasma torch unit of claim 18wherein the current sensing means is characterized by a current sensorcoupled to the work member and to said disconnect circuit, said currentsensor generating a signal in response to current flowing in the workmember to actuate said disconnect circuit.
 21. The plasma torch unit ofclaim 18 wherein said regulated current for the pilot arc is pulsed. 22.The plasma torch unit of claim 18, further characterized by: a pulsingcircuit coupled to said power means for pulsing the pilot arc current.23. The plasma torch unit of claim 22 wherein said pulsing circuitpulses the main arc current.
 24. The plasma torch unit of claim 10wherein: the current regulated power means coupled between theelectrodes and the work member supplies regulated current to the torchelectrode and the pilot electrode for the pilot arc, and to the torchelectrode and the work member for the main arc; and furthercharacterized by a pulsing circuit coupled to the power means andoperable to pulse the current for the pilot arc.
 25. The plasma torchunit of claim 24 wherein said pulsing circuit is operable to pulse thecurrent for the main arc.
 26. A plasma torch unit of the type having atorch assembly adapted to operate on a work member and having a torchelectrode a pilot electrode, and means for providing an ionized gastherebetween; a current regulated power means for supplying regulatedcurrent to the work member and torch electrode during torch usage, saidcurrent regulated power means electrically connected to the work memberby first circuitry provided between the power means and the work memberand electrically connected to the torch electrode by second circuitryprovided between the power means and the torch electrode; pilot arccontrol means for controlling an electric arc between the torchelectrode and the pilot electrode, said pilot arc control meanselectrically connected between the power means and the pilot electrodeand electrically connected to the pilot electrode by third circuitryprovided between the control means and the pilot electrode, said pilotarc control means including disconnect means between the power means andthe pilot electrode; current sensing means for sensing current in saidfirst circuitry and generating a feedback signal in response to torchusage; and diode means connecting between said second and thirdcircuitry to provide a path for the conduct of current to the pilotelectrode; the unit characterized by: first inductor means coupled tosaid pilot arc control means for temporarily sustaining the pilotcurrent when said disconnect means is open; second inductor meansprovided within the power means for forcing the establishment of thetransferred arc; and an interconnect means between the current sensingmeans and the disconnect means so that said current sensing meansoperates said disconnect means with the feedback signal.
 27. The plasmatorch unit of claim 26 wherein the disconnect circuit is connected tosaid pilot arc control circuit and operable to open circuit said pilotarc control circuit to generate an induced voltage in the secondinductor which is greater than the open circuit voltage thereby forcingcurrent to flow between the workpiece and the torch electrode to helpinitiate the transfer of the pilot arc into the main arc.
 28. The plasmatorch unit of claim 26, further characterized by: a current sensorconnected to the workpiece and to said disconnect circuit, said currentsensor generating a signal in response to current flowing in theworkpiece to actuate said disconnect circuit.
 29. The plasma torch unitof claim 27, further characterized by: a pulsing circuit connected tosaid power supply for pulsing the pilot arc current.
 30. The plasmatorch unit of claim 29, wherein said pulsing circuit pulses the main arccurrent.